Hitherto, as the element member of semiconductor devices, photosensitive device for use in electrophotography, image input line sensors, image pickup devices, or other optical devices, there have been proposed a number of amorphous semiconductor film, for instance, an amorphous deposited film composed of an amorphous silicon material compensated with hydrogen atom or/and halogen atom such as fluorine atom or chlorine atom [hereinafter referred to as "A--Si(H,X)"]. Some of such films have been put to practical use.
Along with those amorphous semiconductor films, there have been proposed various methods for their preparation using plasma chemical vapor deposition technique wherein a raw material is decomposed by subjecting it to the action of an energy of direct current, high frequency or microwave glow discharging to thereby form a deposited film on a substrate of glass, quartz, heat-resistant resin, stainless steel or aluminum. And there have been also proposed various apparatus for practicing such methods.
Now, in recent years, the public attention has been focused on plasma chemical vapor deposition process by means of microwave glow discharging decomposition [hereinafter expressed by the abbreviation "MW-PCVD process"] also at industrial level.
One representative apparatus for practicing such MW-PCVD process is such that has a structure as shown in a schematic perspective drawing of FIG. 3.
In FIG. 3, there are shown a whole reaction chamber 301, a substantially enclosed deposition chamber 302, a microwave introducing window 303 which is made of a dielectric material such as alumina ceramics or quartz, a wave guide 304 which transmits microwaves, a microwave power source 305 which generates microwaves, an exhaust pipe 306 being connected through an exhaust valve (not shown) to an exhaust apparatus (not shown), a ring-shaped gas feed pipe 307 being connected through a valve 313 to gas reservoirs (not shown), gas liberation holes 307', a substrate holder 308, a substrate 309 onto which a deposited film is to be formed, an electric heater 310 for heating the substrate, a plasma generation space 311 and microwaves generated from 312 from microwave power source 305.
The deposition chamber 302 ordinarily has a cavity resonant structure so as to resonant with the frequency of the microwave power source 305.
The film forming operation in the apparatus shown in FIG. 3 is carried out in the following way.
That is, the air in the deposition chamber 302 is evacuated by opening the main valve of the exhaust pipe 306 to bring about the space in the chamber to a predetermined vacuum. And the heater 310 installed in the substrate holder is actuated to uniformly heat the substrate 309 to a predetermined temperature and it is kept at that temperature.
Then, raw material gases, for instance, silane gas and hydrogen gas etc. in the case of forming an amorphous silicon deposited film, are introduced into the deposition chamber 302 through the gas feed pipe 307 and its gas liberation holes 307'.
At the same time, microwaves 312 having a frequency of more than 500 MHz, preferably of 2.45 GHz are generated by the microwave power source 305, which is successively introduced into the deposition chamber 302 through the wave guide 304 and the microwave introducing window 303. The raw material gases thus introduced into the deposition chamber 302 are excited and dissociated by an energy of the microwave to generate neutral radical particles, ion particles, electrons and the like and to cause chemical reactions among them resulting in formation of a deposited film on the surface of the substrate 309.
It is conventional for the above-mentioned apparatus to provide at least an opening and shutting member (hereinafter referred to as "removable member") that allows the formation of an opening sufficient enough to or remove the substrate and also to conduct relevant maintenance and inspection of the inside of the deposition chamber. For such removable member, it is required to have an appropriate sealing means which enables one not only to maintain the inside of the deposition chamber in a state of vaccum but also to maintain the gaseous atmosphere of the introduced raw material gases in the deposition chamber.
As such sealing means, there are known, for instance, a metallic gasket and a rubber gasket. Among these gaskets, the rubber gasket is widely used in views of its excellent repeated usability and its sealing reliability.
In FIG. 1 (C), there is shown a schematic partial cross-sectional view for a typical example of such known removable member in the known apparatus for the formation of a functional deposited film by means of MW-PCVD.
In FIG. 1(C), there are shown a microwave plasma generation space 101 of the deposition chamber, a removable member 102 for allowing the formation of the foregoing opening and a circumferential wall portion 103 of the deposition chamber being closely contacted through a vacuum sealing means 105 with the removable member 102 while forming a clearance 104.
For the known MW-PCVD apparatus having such vacuum sealing system as shown in FIG. 1(C), there are problems in that when a microwave is introduced into the plasma generation space 101, part of it often gets into the clearance 104 and then leakes out of the plasma generation space 101 or otherwise. As a result, it becomes absorbed by the vacuum sealing means 105 which then is heated and deteriorates.
Other than the above, there are also other problems in that the vacuum sealing means 105, the inner wall surface of the removable member 102 and the upper wall surface of the circumferential wall portion 103 of the deposition chamber will be contaminated with an undesired deposited film and powdery particles caused by plasmas containing active species resulting from the decomposition of raw material gases during the formation of a desired deposited film on a substrate in the deposition chamber. The removal of these contaminants after completion of the film forming process is difficult even by a proper cleaning means such as dry etching because of the narrowness of the clearance. In addition, in the case where removal of such foreign matters is attempted by means of dry etching, the vacuum sealing means 105 will be deteriorated by an etching energy, a heat caused by plasmas, active species such as fluorine radical, etc.
Further in addition, the known MW-PCVD apparatus is inconsistent in repeatedly producing a deposited film by repeatedly operating it such that the quality of a resulting deposited film will be gradually reduced as it is repeatedly operated. That is because the sealing function of the vacuum sealing means comprising a rubber gasket for example will be decreased as a result of its deterioration caused by the absorption of an microwave and because of this, such foreign matters as mentioned above will be increasingly formed. They eventually intermix with the resulting film to thereby provide an undesirable defect thereon. Microwave plasmas will also enter into the region where said rubber gasket is placed to cause formation of a foreign deposited film. Powdery particles are eventually deposited or adhered onto said rubber gasket or surrounding members to thereby deteriorate the sealing function. Foreign matter attaches to the substrate when it is taken in the deposition chamber or when a raw material gas is introduced into the deposition chamber to thereby invite an undesirable defect on the resulting deposited film.
These problems are aggravated particularly in the case of forming a silicon containing amorphous film such as a film composed of A--Si(H,X) at a high deposition rate by decomposing a silicon atom containing raw material gas such as silane gas (SiH.sub.4) using the foregoing apparatus for such reasons that a high microwave energy is used and powdery particles of polysilicon and the like are likely to generate in this case. These problems will often occur even in the case where a substrate small in size is used.
For instance, in case of a photosensitive member for use in electrophotography, since its light receiving layer is usually such that has a thickness of, for example, 20 to 40 .mu.m, it takes a long period of time for the formation thereof. Because of this, should the foregoing problems occur during the film forming process, the resulting layer is accompanied with such defects to make the resulting photosensitive member practically inapplicable.